The present invention relates to signal processing systems, for example, hybrid infrared charge-coupled devices. More particularly, this invention relates to an interface structure between a signal processor and an associated signal source. In many contexts, there is a problem matching a signal to be processed with the characteristics of the device selected to process that signal. As a case in point, the performance of a state-of-the-art hybrid infrared charge-coupled device (IRCCD) is limited by the charge handling capacity of the incorporated charge-coupled device (CCD). This limitation proves to be a major obstacle in approaching performance goals such as background-limited performance (BLIP), which refers to a signal-to-noise ratio (S/N) limited to that inherent in the scene being viewed.
A hybrid IRCCD comprises two integrated circuits which are fabricated separately, then physically and electrically connected. One of these integrated circuits constitutes a detector array and a CCD. The detector array can include thousands of detector elements each adapted for providing a time-varying output as a function of incident infrared radiation. In many IRCCDs, each detector element output is directly injected into a CCD input where it is converted into packets of electric charge for storage and processing. For example, the CCD can combine, compare and multiplex the detector outputs, depending on the application. Operationally, a CCD controls local electric fields to transfer charge and to effect the storage of such charge in "potential wells", also referred to more quaintly as "charge buckets". These charge buckets, like their liquid bearing counterparts, have limited capacity. This capacity can be exceeded by an excessive current at a CCD input, or by the internal combination the contents of two partially full potential wells. The combination of the two packages results in a saturation condition. From a system point of view, exceeding the capacity of the potential wells results in elimination of the signal.
The limited charge-handling capacity makes it impractical to use the direct injection input structure with photovoltaic detectors in the long wavelength region. With photoconductive detectors, the bias could be lowered, but this would imply a lower photoconductive gain, a smaller signal, and, therefore, a decreased signal-to-noise ratio. Instead of injecting detector current into the CCD, a gate modulation approach uses the detector current to generate a variable voltage at a CCD transfer gate to modulate a current induced by a constant voltage source. This approach does result in a better match to the CCD's charge handling capacity without unduly sacrificing sensitivity or S/N. However, this requires in addition to a contact at the injection input, a contact at a gate input. The additional closely-spaced input requires greater process complexity, a larger area per input, and, thus, a lower process yield. It is found that the gate modulation input is susceptible to non-uniformities due to input threshhold variations.
What is needed is a practical signal processor interface structure which allows better use of the charge capacity of the signal processor without an offsetting decrease in sensitivity. In the context of hybrid IRCCDs and other CCD-based devices, the structure avoid unduly complicating the manufacture of the CCD which already strains the limits of processing technology.